1. Field of the Invention
The present invention relates to an electromagnetic-field-strength reducing device, an electromagnetic-field-strength reducing method, and a radio communication device which are preferably applied to radio communication equipment having a portion that is brought close to a human ear. Examples of such radio communication equipment include a mobile phone, a PHS (personal handyphone system) phone, a PDA (personal digital assistant) device, a headphone device, a portable game console, and a music player device.
In particular, the present invention relates to an electromagnetic-field-strength reducing device, an electromagnetic-field-strength reducing method, and a radio communication device, which can reduce the electric strength and the magnetic-field strength of a portion of a radio communication device, the portion being brought close to the ear, and which can comply with a standard for a method for measuring compatibility between wireless communication devices and hearing aids, specifically, the HAC (Hearing Aid Compatibility) standard (ANSI C63.19) standardized by the Federal Communications Commission (FCC).
2. Description of the Related Art
Today, digital wireless devices, such as mobile phones, have been widely used. Voice during a phone call with such a digital wireless device is transmitted through a wireless network over radio waves. During a phone call, radio waves (radio frequency emissions) output from the digital wireless device form an electromagnetic field having a pulsed pattern around the antenna of the digital wireless device.
The electromagnetic field may produce a buzzing noise when picked up by a microphone section or a telecoil circuit of the hearing aid, and can adversely affect the wearer of a hearing aid. Thus, it is preferable that the strength of the electromagnetic field (electric field) formed by the digital wireless device be reduced to a level that does not adversely affect the wearer of the hearing aid.
Given this situation, the FCC sets the HAC standard (ANSI C63.19) regarding a method for measuring compatibility between wireless communication devices and hearing aids.
The HAC standard specifies a unified measurement method and parametric requirements regarding electromagnetic compatibility, operation compatibility, and accessibility between hearing aids and digital wireless devices, such as, mobile phones, cordless phones, and VoIP (Voice over Internet Protocol) devices that which operate in a frequency range of 800 to 950 MHz and a frequency range of 1.6 to 2.5 GHz. Accordingly, it is important for the manufactures of the digital wireless devices, such as mobile phones, to manufacture products that comply with the HAC standard.
Through patent research of related art of the present invention, the assignee of the present invention has found the technology disclosed in Japanese Unexamined Patent Application Publication No. 2002-353719.
Japanese Unexamined Patent Application Publication No. 2002-353719 discloses a SAR (specific absorption rate) reducing device for reducing a local averaged SAR by reducing the amount of radiation of electromagnetic waves. In the SAR reducing device, a radiation reducing section that serves as a portion for reducing the amount of electromagnetic wave radiation is provided at an installation surface of a casing and a current source for supplying current is provided at a reverse side of the casing, the reverse side being opposite to the installation surface.
The SAR reducing device has a conductive member that resonates at a frequency used for communication. The conductive member is provided so as to oppose the reverse side of the casing and has an open lower end. In the SAR reducing device, the upper end of the conductive member is connected to ground, located in the vicinity of the current source at the reverse side of the casing, via a conductive short-circuiting member.
In such a SAR reducing device, during communication, the conductive member resonates at a frequency used for the communication. In this case, the impedance of the open lower end of the conductive member becomes ∞ (infinite), and the impedance of the upper end of the conductive member, the upper end being connected to ground, approaches zero.
Thus, since the upper end of the conductive member is connected to the vicinity of the current source, current supplied from the current source flows to the conductive member. Thus, the SAR reducing device can reduce the amount of current flowing from the current source to the emission reducing section and thus can reduce the amount of electromagnetic wave emission from the emission reducing section.
In addition, since the upper end of the conductive member is attached to the reverse side of the casing, the SAR reducing device facilitates provision of the conductive material without ensuring a space for disposing the conductive member on the installation surface of the casing.